To facilitate the need for smaller contact geometries in VLSI devices, transition-metal silicides have been used to provide a highly-conductive body having a shallow junction depth in a silicon substrate. As both the contact size and junction depth are reduced, a new device metallization processes are required to overcome the problems which are encountered.
One such recent device metallization process is the self-aligned silicide process (salicidation). In the salicidation process, a transition-metal layer is deposited to overlie device regions to which an electrical connection are to be made. The transition-metal layer also overlies insulating layers on the surface of the device substrate. The substrate is then annealed at a high temperature to form a conductive silicide layer at the surface of the device region. The silicide is formed by a reaction between the transition-metal, and silicon in the underlying device region. In areas in which the transition-metal layer overlies a dielectric material, no substantial chemical reaction occurs between the transition-metal and the dielectric material. Following the silicide forming reaction, remaining portions of the transition-metal which did not react with silicon are removed. Thus, an electrically conductive silicide material is formed only in areas where the transition-metal directly contacts a silicon device region. The salicidation process avoids the need to pattern the transition-metal prior to forming a silicide layer, making the formation of smaller electrical contacts possible.
Typically, either titanium or cobalt are used as the transition-metal in the salicidation process. These transition-metals readily react with silicon to form titanium silicide (TiSi.sub.2), and cobalt silicide (CoSi.sub.2), and the silicides of cobalt and titanium have good thermal stability. Additionally, these transition-metals can be readily deposited by existing, well characterized metal deposition processes such as physical and chemical vapor deposition.
Typically, the annealing process used to form the transition-metal silicide is carried out in either a conventional furnace, or a rapid thermal annealing apparatus. It is during the annealing process that the transition-metal reacts with silicon to form the transition-metal silicide. To avoid oxide formation during the reaction, the annealing system is purged and nitrogen, or another inert gas, is introduced to the annealing system. In the case of titanium, the titanium also reacts with nitrogen and forms titanium nitride (TiN). Although titanium nitride is electrically conductive, the formation of titanium nitride reduces the amount of titanium available for reaction with silicon to form titanium silicide. Additionally, any oxygen present in the annealing system can react with the transition-metal to form a transition-metal oxide. Also, oxygen can diffuse, to a certain extent, through the transition-metal to the silicon interface and react with silicon to form silicon dioxide. Either of these reactions produce an electrically insulating material which increases the contact resistance in the device regions. The necessity of removing even trace amounts of oxygen from the annealing system requires a leak proof system and a high flow rate of very pure nitrogen gas during the annealing process. The stringent process conditions result in high processing costs and poor repeatability of the silicide formation process.
To reduce the adverse effects associated with unwanted transition-metal reactions, a capping layer can be used to overlie the transition-metal layer. A capping layer can reduce the exposure of the transition-metal to ambient gases and oxygen during the silicide formation process. To be effective, the capping layer must not substantially react with the underlying transition-metal layer, and must be easily removable following the silicide formation process. Additionally, the capping layer must provide a diffusion barrier to oxygen and ambient gas molecules. Both insulating and conductive materials have been used as capping layers in silicide formation processes. For example, silicon dioxide, deposited using tetraethylorthosilane (TEOS), can be formed on a transition-metal layer to prevent a reaction between ambient gases and the transition-metal. However, the oxygen in silicon dioxide can react with the transition-metal and form an oxide at the interface between the transition-metal and the silicon dioxide. Additionally, the TEOS layer must be removed with a highly selective etch which will not damage the underlying transition-metal silicide.
An electrically conductive material such as titanium nitride can also be used as a capping layer. Titanium nitride provides a good barrier to the diffusion of ambient gases to the silicon surface during the silicide formation process. However, titanium nitride can react with oxygen to form titanium oxynitride, which is difficult to remove. Also, titanium oxynitride reduces the diffusion barrier capability of the titanium nitride layer. Accordingly, further development of capping layers is required to provide a capping layer which can be easily removed following silicide formation, and which provides a barrier to the diffusion of both oxygen and ambient gases during the silicide formation process.